A new study confirms that the reaction time of cell phone users slows dramatically, increasing the risk of accidents and tying up traffic in general, and when young adults use cell phones while driving, they're as bad as sleepy septuagenarians.

"If you put a 20-year-old driver behind the wheel with a cell phone, their reaction times are the same as a 70-year-old driver who is not using a cell phone," said University of Utah psychology professor David Strayer. "It's like instantly aging a large number of drivers."

Traffic jams and death

Cell phone distraction causes 2,600 deaths and 330,000 injuries in the United States every year, according to the journal's publisher, the Human Factors and Ergonomics Society.

The reason is now obvious:

Drivers talking on cell phones were 18 percent slower to react to brake lights, the new study found. In a minor bright note, they also kept a 12 percent greater following distance. But they also took 17 percent longer to regain the speed they lost when they braked. That frustrates everyone.

Older drivers more cautious

The latest study used high-tech simulators. It included people aged 18 to 25 and another group aged 65 to 74. Elderly drivers were slower to react when talking on the phone, too.

The simulations uncovered a twofold increase in the number of rear-end collisions by drivers using cell phones.

Older drivers seem to be more cautious overall, however.

"Older drivers were slightly less likely to get into accidents than younger drivers," Strayer said. "They tend to have a greater following distance. Their reactions are impaired, but they are driving so cautiously they were less likely to smash into somebody." But in real life, he added, older drivers are significantly more likely to be rear-ended because of their slow speed.

Other studies in the journal found:

Telephone numbers presented by automated voice systems compete for drivers' attention to a far greater extent than when the driver sees the same information presented on a display.

Interruptions to driving, such as answering a call, are likely to be more dangerous if they occur during maneuvers like merging to exit a freeway.

Things could get worse. Wireless Internet, speech recognition systems and e-mail could all be even more distracting.